Chapter 2 The Sea Floor

Sea Floor Geologically distinct from the continents Perpetual cycle of birth and destruction that shapes the oceans and controls the geology and geological history of the continents

Sea Floor Processes Occur slowly (hundreds of millions of years) Solid rocks flow like liquid Entire continents move over the face of the earth

Geology is Important to the Marine Biologist Habitat – natural environment that an organism lives Habitats are shaped by geological processes

Geological Processes Determine: The Form of coastlines The depth of water Type of bottom (muddy, sandy or rocky)

The Water Planet Presence of water makes earth unique Oceans cover 71% of the globe Regulate our atmosphere and climate Life would be impossible without water

The Geography of the Ocean Basins

2/3 of land area is in Northern Hemisphere 61% of N. Hemisphere is ocean 80% of Southern hemisphere is ocean

Ocean Basins 4 large ocean basins Pacific, Atlantic, Indian, Artic

Pacific Deepest and largest

Atlantic and Indian Atlantic is a little bit bigger Similar in average depth

Artic Smallest Shallowest

The Oceans Are all interconnected Described as a single world ocean Southern Ocean – continuous body of water that surrounds Antarctica

Figure 2.01

The Structure of the Earth

Earth originated 4.5 billion years ago from clouds of dust Dust left over From Cosmic explosion (Big Bang) which occurred 14 million years ago Dust collided and made bigger particles which collided and made bigger particles until planets were formed Fusion of particles

Heat was generated which made earth molten allowing the materials to settle by density Density – mass of a given volume of a substance D = M/V

Light surface material cooled into a thin crust Eventually the atmosphere and oceans formed Earth settled into orbit at a distance that allows liquid water to exist and therefore life as we know it

Internal Structure of Earth Concentric layers based on density (like and onion) Core, mantle, crust

Core Inner most layer Alloys of iron Pressure is more than a million times greater than at the surface of earth 4000 o C Solid inner core Liquid outer core

Magnetic Field Swirling motions of the liquid material in the iron-rich outer core produce the earth’s magnetic field

Mantle Solid Very hot – near melting Flows almost like a liquid, but much slower Swirls and mixes like very thick soup heating in a saucepan

Crust Outermost layer Extremely thin Rigid skin floating on top of the mantle

Figure 2.03

Continental and Oceanic Crusts

Geological distinction between ocean and continents results from physical and chemical differences in the rocks themselves

Oceanic Crust Makes up the sea floor Basalt rock – dark color More dense Thinner

Continental Crust Granite rock – light color Less dense thicker

The continents can be thought of as thick blocks of crust floating on the mantle much as icebergs float in water Oceanic crust floats on the mantle too, but because it is denser it does not float as high

Ages of the Crust Oceanic rocks are less than 200 million years old Continental rocks can be 3.8 billion years old

The Origin and Structure of the Ocean Basins

Early Evidence of Continental Drift 1620 – Sir Francis Bacon – coasts of continents on opposite sides of the Atlantic fit together like pieces of a puzzle Coal deposits and other geological formations match up Fossils from the different coasts are similar

1912 – Alfred Wegner – proposed first detailed hypothesis of continental drift Continents were joined as a single “super continent” called Pangea

The Theory of Plate Tectonics Wegner could not explain how the continents could move so his theory was not well accepted 1950’s and 1960’s evidence was put together that proved that continents did drift The process involves the entire surface of our planet – plate tectonics

Discovery of the Mid-Ocean Ridge After WW II sonar allowed the first detailed surveys of large areas of the sea floor Lead to the discovery of the Mid-ocean Ridge A continuous chain of submarine volcanic mountains that encircles the globe like the seams of a baseball

Along the ridge at regular intervals there are cracks or Faults (transform faults) in the earth’s crust Occasionally the ridge comes out of the ocean to form islands like Iceland

Mid-Atlantic Ridge Mid-ocean ridge in the Atlantic Runs down the center of the Atlantic ocean

Eastern Pacific Rise Main section of the ridge in the eastern pacific

Trenches Deep depressions in the sea floor Especially common in the Pacific

Figure 2.05

Significance of the Mid-Ocean Ridge

Earth quakes are clustered around the ridge Volcanoes are concentrated near the trenches

Glomar Challenger Drilled samples of the deep-sea floor Samples revealed that the sea floor was young especially when compared to the continents Mid ocean ridge crest had the youngest rock Rocks get progressively older as you move away from the ridge crest

There is little sediment at the ridge crest but it becomes increasing thicker as you move away

Magnetism of Ocean Floor Rocks It was known that earth's magnetic field reverses direction every few million years Many rocks contain tiny magnetic particles When a rock is molten these particles can move When the rock solidifies the particles are frozen in place and keep their orientation

Geologists found patterns of magnetic bands or stripes in the sea floor running parallel to the mid-ocean ridge The bands are symmetric around the ridge Magnetic bands = magnetic anomalies

Figure 2.08

Significance The bands of normally magnetized sea floor must have formed at different times from their reverse-magnetized bands So the sea floor was not formed all at once but in strips the parallel the mid- ocean ridge

Figure 2.09

Creation of the Sea Floor

Sea Floor Spreading Huge pieces of oceanic crust are separated at the mid-ocean ridges creating cracks or rifts in the crust When a rift occurs, pressure is released and hot mantle material rises up through the rift This molten rock pushes the oceanic crust up to form the mid-ocean ridge and new oceanic crust Therefore the rifts are known as spreading centers

Sea Floor Spreading Explains: Observations relating to the mid-ocean ridge Sediment build up Age of the rocks Magnetic stripes

Sea-Floor Spreading and Plate Tectonics

Lithosphere Made of the crust and the upper mantle 100 km thick or 60 miles thick “rock sphere” Broken up into plates – lithospheric plates

Lithospheric Plates Made of continental crust, oceanic crust or both The lithosphere floats on a denser, more plastic layer of the upper mantle known as the asthenosphere

Asthenosphere vs. Lithosphere Distinction is made on how easy the rock flows The swirling motions of the asthenosphere drives the motion of the lithospheric plates

Figure 2.15

Continental Drift Mid-ocean ridges form the edges of many of the plates Lithospheric plates move apart and new sea floor is created Mechanism for continental drift Plates move apart about 2 to 18 cm a year (.8 to 7 in) (fingernails grow 6 cm or 2.4 inches a year)

Destroying Lithosphere As new lithosphere is created old lithosphere is destroyed This occurs in the trenches When two plates collide, one of the plates dips below the other and sinks back down into the mantle – Subduction (downward movement of the plate)

As the plate moves downward is melts As the plate breaks apart earthquakes can happen The new extra molten material can rise back to the surface to form volcanoes

Oceanic plate with a continental plate Oceanic plate goes under the continental plate Continental plate is less dense Explains why old rocks are only found on continents Oceanic crust is always destroyed in the trenches so it never gets old Volcanoes are often associated with the trench

Figure 2.11

Oceanic with an oceanic One dips beneath the other to form a trench The trench is associated with earthquakes and volcanoes Volcanoes can rise from the sea and form islands Trenches are curved because of earth’s spherical shape Islands follow this curvature and form island arcs Ex. Aleutian and Mariana islands

Figure 2.12

Continental with a Continental Both plates tend to float and neither is subducted The two plates push against each other with such force that they become “welded” together The force eventually becomes too great and the rock buckle and fold like an accordion

The huge folds form mountain ranges Ex. Himalayas

A fourth boundary Two plate can move in such a way that they slide past each other Lithosphere is neither created nor destroyed Shear boundary Immense friction between the plates Plates lock, stress builds and then suddenly break free and slip causing and earthquake Ex. San Andres Fault - California

Margins Active margin – type of continental margin where one plate is colliding with another plate as a result of geological activity – step rocky shores, little sediment Passive margin – continental margin that is located at the trailing edge of a continent and as a result shows little geological activity – flat, lots of sediment, wide continental shelf

Figure 2.22

Figure 2.23

Figure 2.24

Dynamic Mantle

Hot spot Found in about 45 places around the world Hot, molten rock or magma well up from deep within the mantle This magma forces its way up through the lithosphere Erupts in volcanic activity

Hot Spot Examples Geysers and bubbling mud pools at Yellowstone result from volcanic activity Seamounts – volcanic underwater mountains Hawaiian islands were created from hot spots – as the plate moved new islands formed Island chains in the south pacific

Hot spots by mid-ocean ridges also form islands – Ex. Iceland, Azores and the Galapagos islands

Text Art 2.02

Geological Provinces of the Ocean

Figure 2.19

Sea Floor Divided into two main regions Continental margins – submerged edges of the continents Deep-sea floor itself

Continental Margins Boundaries between continental crust and oceanic crust Sediments from land accumulate here (can be as thick as 10 km or 6 mi) Shallow, gently sloping region (continental shelf) Steeper area (continental slope) Gently sloping region (continental rise)

Continental Shelf Shallowest 8% of the oceans surface Biologically the richest part of the ocean (most life and best fishing) Submarine canyons – remnants of rivers and glaciers that once flowed across the continental shelves

Varies in width from less than 1 km (.6 mi) to 750 km (470 mi) Shelf ends at the shelf break where the slopes gets abruptly steeper Shelf break usually occurs at depths of 120 to 200 m (400 to 600 ft)

Continental Slope Closest thing to the exact edge of the continent Begins at the shelf break and descends downward to the deep sea floor

Continental Rise Deep sea fan – sediment moving down a submarine canyon accumulated at the canyon's base forms a deep sea fan (like a river delta) Rise consists of a thick layer of sediment piled up on the sea floor

Figure 2.20

Deep Ocean Basins

Deep Sea Floor Depth of 3, ,000 m (10,000 to 16,500 ft.) Abyssal plain Rises at a very gentle slope towards the ridge

Geological Features of the Abyssal Plain Submarine channels Low abyssal hills Plateaus, rises and other features Seamounts (submarine volcanoes) Guyots – flat topped seamounts

Trenches Plate descends into the mantle Sea floor slopes steeply downward Deepest parts of the world ocean Mariana Trench – Western Pacific – 11,022 m or (36,163 ft) deep

The Mid-Ocean Ridge and Hydrothermal Vents

Figure 2.25

Plates are pulling apart at the ridges This leaves a great gap known as the center rift valley Seawater seeps down into this crack and gets heated to high temperatures Heated water forces its way back up through the crust and emerges in hydrothermal vents or deep-sea hot springs

Water is 10 to 20 o C (50 to 68 o F) warmer than the surrounding water Some vents can have water as hot as 350 o C (660 o F)

As the hot water seeps through the cracks it dissolves a variety of minerals, mostly sulfides As the water comes out it is cooled rapidly and the minerals solidify forming mineral deposits around the vents

Black Smokers One type of mineral deposit found at hydrothermal vents Chimney-like structures that progressive build up around a vent as the minerals solidify “smoke” is actually a dense cloud of mineral particles

Life around the vents There is a rich diversity of marine life around the hydrothermal vents One of the most exciting finds in the history of marine biology

The End ……